1. Field of the Invention
The present invention relates to the measurement of isometric and isotonic contraction of blood vessels. More particularly, the present invention relates to devices, systems and methods for isometric and isotonic contraction of blood vessels using an isovolumic myograph.
2. Background of the Invention
Vascular smooth muscle coils (VSMCs) modulate the tone of a blood vessel in response to neural, humoral or local hemodynamic stimuli. The VSMCs are important for auto-regulation and largely determine the spatial and temporal distribution of blood flow in an organ. Thus, conditions that affect the proper function of VSMCs cause a variety of medical problems.
Many diseases, including hypertension, diabetes, heart failure and atherogenesis, show signs of impaired arterial vasoactivity. Hypertension, for example, is identified in relation to changes in the myogenic tone of the resistance arteries. The vasoactivity may be attenuated due to physiological (normal growth, exercise, pregnancy, etc.) or pathological remodeling (hypertension, hypertrophy, heart failure, etc.). The pressure-induced myogenic response (or tone) is initiated as a consequence of pressure-dependent modification of vascular smooth muscle wall tension and subsequent activation of mechanosensitive ion channels. Steady-state myogenic tone accounts for a substantial portion of the peripheral resistance and is an important determinant of arterial blood pressure. Although vasoconstriction and vasodilation are intrinsic properties of VSMC, they are often modulated by endothelium-derived vasoactive factors.
Because of the importance of maintaining proper vasoactivity in VSMC, various drugs are tested for their effects on such vasoactivity. Two of the tools used in such tests to identify vasoactivity in blood vessels include the wire and pressure myographs. A Medline search with keyword “wire myograph” or “pressure myograph” reveals 140 and 207 publications, respectively, from 1990 to the present having at least some reference to these conventional tools for testing vasoactivity. In pharmacology, these methods are used to understand the vasoreactivity and the dose-response relation of various agonists and antagonists.
Although the wire myograph method is used often for pharmacological experiments, it has a number of drawbacks, one being that it is far from physiological. The mechanical deformation of the ring is non-physiological and the cutting of the vessel produces some injury to the vessel which has a direct impact on the response of the vessel to the testing.
The pressure myograph was developed to address some of the limitations of the wire myograph. In the pressure myograph, the vessel geometry and loading are typically more physiological. The pressure myograph method involves changes in pressure while recording the change in diameter under passive and active conditions. The method is substantially isobaric because the pressure is maintained constant during contraction. Since the radius changes during the test, which can change the wall stress (based on Laplace's equation), this method of mechanical testing is neither isometric nor isotonic, which in turn affects interpretation of the results.
Thus, although both of the above conventional methods are widely in use, a need exists in the art for an alternative to the conventional techniques for testing vasoactivity in blood vessels such that the need addresses the setbacks and limitations of the conventional techniques, while at the same time, is easy to use and interpret and provides a more accurate measurement of vasoactivity.